Glucose transporter gene expression in lactating bovine gastrointestinal tract

Absorption of glucose from the intestine and probably from various portions of the gastrointestinal tract (GIT) in lactating dairy cows can have profo und implications because the availability of glucose, or glucose precursors , seems to be a limiting factor for milk synthesis. This article reports on the expression of the Na+/glucose cotransporter gene (SGLT1) in mucosal ti ssues from rumen, omasum, abomasum, duodenum, jejunum, ileum, and cecum of lactating cows. Sodium/glucose cotransporter mRNA transcripts were detected in polyadenylated RNA of all the GIT tissues examined and in total RNA fro m rumen, omasum, duodenum, jejunum, ileum, and cecum. The pattern of SGLT1 mRNA distribution in these tissues was different from that of the facilitat ive glucose transporter GLUT5. Sodium/glucose cotransporter protein was als o detected in small intestinal membranes of lactating dairy cows. Sodium-de pendent glucose uptake across small intestinal brush border membrane vesicl es could be resolved into linear nonsaturable (diffusion) and saturable, ca rrier-mediated components. The diffusion constant of glucose from jejunal m embrane vesicles was 18.3 +/- 2.1 pmol.mM(-1).mg protein(-1).s(-1). The sat urable component of glucose uptake had a V-max of 27.1 +/- 5.8 pmol.mg prot ein(-1).s(-1) and a K-m of .12 +/- .05 mM. Similar transport kinetics were found in the duodenum, and they were less in the ileum. These data suggest that the GIT of lactating cows is capable of active transport of glucose fr om the lumen across the brush border membrane of the epithelial cells. In a ddition, the relatively high abundance of SGLT1 mRNA in the rumen, omasum, and cecum may imply new functional and nutritional roles for these tissues in the active transport of glucose.